1. Field of the Invention
This invention relates to x-ray tubes and, more particularly, to high-power x-ray tubes having increased average power dissipation.
2. Prior Art
X-ray tubes have applications in two fields: medical x-ray diagnostic imaging and technical x-ray imaging. Medical imaging x-ray tubes are characterized as providing x-rays from a high brightness focal spot, and with a low duty-cycle. Technical x-ray tubes, which are used, for example, for non-destructive testing (NDT) are characterized as providing x-rays from a focus with lower brightness but with high duty-cycles. Most medical x-ray tubes use a rotating target anode enclosed within a vacuum envelope to achieve high peak brightness. The rotating anode is often a disk made from a high melting point metal, which is cooled by high-temperature radiation cooling. X-rays are generated by accelerating electrons onto the target (anode). The yield for x-ray generation is so low that about 99% of the electron beam power is converted into wasted heat energy. Failure to dissipate this heat results in a temperature rise which can irreversibly damage or destroy components of these expensive tubes. The efficiency of radiation cooling dramatically increases at higher temperatures so that efficient radiation cooling requires operation of the anode at high temperatures, which increases the conditions for and the likelihood of tube damage or failure. In contrast, technical x-ray tubes use a fixed target anode which can be cooled by direct contact with a cooling fluid, permitting high duty-cycles at low energy.
Medical x-ray tubes are used in computerized-tomography CT imaging systems as a source of high brightness, narrowly focused x-rays to precisely measure attenuation data, which then are "reconstructed" to form images for medical diagnosis. However, CT imaging systems, or scanners, have severe operational limitations imposed upon them due to the limited duty-cycles of the rotating anode x-ray tubes used in such CT imaging systems. In operation, because commercial x-ray tubes used in CT systems have very low duty-cycles, these CT systems must be used intermittently so that the x-ray tube can cool down to a safe operating temperature. For example, a typical abdominal scan requires 20,000 watts of electron beam power. Yet, the maximum power dissipation of a typical rotating-anode x-ray tube is in the range of 100 watts, with 2000 watts power dissipation being available for certain tubes employing an oil-recirculating heat-exchanger. This results in an effective duty cycle of 0.005 to 0.1, being careful not to exceed the maximum power dissipation temperature limits.
One component which is particularly subject to damage and failure is the bearing for supporting the rotating anode of the x-ray tube within the vacuum envelope. Typically, the anode disk is mounted at the end of a rotatable structure supported by the bearing. The bearing surfaces are contained with the vacuum of the tube. Because a typical lubricant would contaminate the vacuum enclosure, no such lubricants are used. Heat dissipation from a tube during high load conditions is provided primarily by radiation of thermal and optical radiation energy from the rotating anode disk to the walls of the envelope containing the vacuum for the tube. The walls of the envelope are composed of glass, metal, and/or ceramic materials and may be surrounded by a dielectric oil bath. For radiation cooling to be effective, the anode disk must be at an elevated temperature. During high loading conditions, the anode disk does get to a high temperature and cooling becomes more efficient. However, if the anode temperature is elevated for an extended period of time, the bearing gets too hot and its lifetime is dramatically reduced. With the advent of CT, the designs of existing rotating x-ray tubes were challenged. Bearings were redesigned to prevent movement of the focal spot, that is the region on the anode struck by an electron beam, as components of the tube expanded and contracted as the temperature of the tube changed. CT systems were particularly sensitive to movement of the focal spot on the target anode.
Another challenge to tube designers was to increase the number of CT scans before the tube had to be idled for cool-down. It appears that most tube and equipment manufacturers have chosen the same solution to this problem: Increase the heat capacity of the rotating anode structure. The amount of heat which can be stored in the anode up to its maximum allowed temperature is commonly called "heat-loadability" of the tube. Its value is commonly given in watt-seconds (joules) or in "heat units" (one joule is approximately 1.3 heat units). These solutions have involved increasing the diameter, size, weight, and surface emittance of the rotating anode disk, as well as using heat-exchangers for the oil-dielectric surrounding the vacuum envelope of these tubes. Not much progress has been made regarding the bearings contained within the vacuum.
Currently, the newest and largest heat capacity rotating x-ray tubes being commercially produced use heat exchangers and can dissipate approximately 3000 watts. Since continuous input powers of 20-30,000 watts are still desired, these x-ray tubes have a duty-cycle of approximately 10% and still must be kept idle for over 90% of the time. Actual operation at a power level of 3000 watts would reduce the life of their bearings to a few days or even hours. In addition, these tubes with their associated heat-exchangers are quite bulky and very expensive.
Even though x-ray tube designs have been incrementally improved, it still remains a problem that the type of x-ray tubes needed for CT still need to be idled once they have their thermal capacity loaded up by initial operation of the tube from a cold start. In the operation of a CT system, a certain amount of this type of idle time can be masked partially by whatever time is required to perform digital data processing and image reconstruction. As electronic computer processing systems become faster and less expensive, image reconstruction times become shorter and soon will be the same as the actual x-ray scanning time. Thus, the x-ray tube is the limiting factor when higher patient throughput is needed, for example, to improve the economic balance sheet of a facility, to cope with civil emergency situations, and to handle battlefield triage conditions.
Most technical x-ray imaging systems do not use rotating anode tubes. These systems use so-called stationary anode tubes, which are rugged tubes normally operated at up to a 100% duty cycle and which have substantial service lives. This type of tube has a stationary, liquid-cooled anode. However, their peak power is rated at only approximately 2% of the peak power of a rotating anode tube used in medical imaging systems. Since the focal spot remains stationary on the target anode, the power of a stationary anode tube is limited typically to 300 watts for an effective focal spot size of 1 by 1 millimeter to 50 watts for a 50 micrometer diameter focal spot size. For applications requiring high spatial resolution, a small focal spot is required and the tube power must be correspondingly reduced. Because their peak power is low, these tubes in addition to having severe limitations with respect to their spatial resolution capabilities, have limitations on the maximum allowed thickness of the object to be tested. Whereas digital image acquisition processing and display-methods have been introduced in medical imaging over the past 15 years, technical X-ray imaging is still mostly done with silver-based photographic film. One of the reasons that there is so little progress in digital X-ray imaging for technical application is believed to be the low brightness of the focal spot of stationary-ray tubes. For X-ray films this is not a problem because they are ideal integrators for x-ray photons and by simply increasing the exposure time (in some cases to as much as hours or longer), the low brightness of the technical tubes can be accounted for. Modern digital (electronic) imaging devices however require a certain minimum x-ray flux for recording because the signal level is required to be above the noise floor of the electronic x-ray detection device. An x-ray tube which would combine the high flux of a rotating tube with the high duty cycle of a stationary tube would make it possible for digital imaging to enter the field of technical x-ray imaging.
A number of improved bearings have been proposed for rotating anode x-ray tubes. Also, rotating x-ray tubes are available which use fluid-cooling of the rotating anode, such as, for example, tubes provided by Elliot of England and Rigaku of Japan. These tubes do combine the strong point of the rotating anode tubes (higher peak power capacity) with the strong point of the fixed anode tubes (direct fluid-cooling of the anode). However, these tubes are not used in medical imaging systems because the peak performance of these tubes is not equal to that provided by current rotating anode tubes. In addition, these tubes have another disadvantage which is that they are not hermetically sealed. The rotating shaft for the anode goes through the vacuum envelope via a rotary seal which uses a magnetic fluid with a low vapor pressure. The tube needs to be connected to a vacuum pump to maintain and/or establish a high vacuum within the envelope of the tube. This significantly increases the complexity and cost of an imaging system in addition to decreasing reliability.